1. Introduction
This invention relates to novel photoactive compounds and to photoresists using said photoactive compounds. More particularly, this invention relates to positive working photoresists that possess light attenuating capability when exposed to near ultraviolet to visible radiation and use an esterification product of a diazooxide and curcumin dye as the photoactive compound.
2. Description of the Prior Art
Photoresist compositions are known in the art and are described in numerous publications including De Forest, Photoresist Materials and Processes, McGraw-Hill Book Company, New York, 1975. Photoresists comprise coating produced from solution or applied as a dry film which, when exposed to light of the proper wavelength, are chemically altered in their solubility to certain solvents (developers). Two types are known. The negative acting photoresist is initially a mixture which is soluble in its developer but following exposure to activating radiation, becomes insoluble in developer thereby defining a latent image. Positive working photoresists work in the opposite fashion, light exposure making the resist soluble in developer.
Positive working photoresists are more expensive than negative working photoresists but are capable of providing superior image resolution. For example, positive working photoresists can be developed to yield relief images with a line width as low as 1 micron or less. In addition, considering the cross section of a photoresist image, the channels formed in the photoresist by development have square corners and side walls with only minimal taper.
Most positive working photoresists comprise a photoactive compound in a film forming polymeric binder. Photoactive compounds or sensitizers, as they are often called, most frequently used are esters and amides formed from o-quinone diazide sulfonic and carboxylic acids. These esters and amides are well known in the art and described by De Forest, supra, pages 47 through 55, incorporated herein by reference. These light sensitive compounds, and the methods used to make the same, are all well documented in prior patents including German Pat. No. 865,140 and U.S. Pat. Nos. 2,767,092; 3,046,110; 3,046,112; 3,046,119; 3,046,121; 3,046,122 and 3,106,465. Additional sulfonamide sensitizers that have been used in the formulation of positive working photoresists are shown in U.S. Pat. No. 3,637,384. These materials are formed by the reaction of a suitable diazide of an aromatic sulfonyl chloride with an appropriate resin amine. Methods for the manufacture of these sensitizers and examples of the same are shown in U.S. Pat. No. 2,797,213. Other positive working diazo compounds have been used for specific purposes. For example, a diazo compound used as a positive working photoresist for deep U.V. lithography in Meldrum's diazo and its analogues as described by Clecak et al, "Technical Disclosure Bulletin," Vol. 24, No. 4, September 1981, IBM Corp., pp. 1907 and 1908. An o-quinone diazide compound suitable for laser imaging is shown in U.S. Pat. No. 4,207,107.
The resin binders most frequently used with the o-quinone diazides in commercial practice are the alkali soluble phenol formaldehyde resins known as the Novolak resins. Photoresists using such polymers are illustrated in U.K. Patent No. 1,110,017. These materials are the product of a reaction of a phenol and formaldehyde under conditions whereby a thermoplastic polymer is formed with a melting point of about 125.degree. C. Novolaks with melting points well in excess of 125.degree. C. are known but are not generally used in photoresist formulations because they are often brittle or have other properties limiting their use.
Another class of binders used with the o-quinone diazides are the homopolymers and copolymers of vinyl phenol. Photoresists of this nature are disclosed in U.S. Pat. No. 3,869,292. It is believed that photoresists using binders of polymers formed from vinyl phenols have not been used in commerce.
Another positive resist known to the art utilizes a resin that is a partially aqueous soluble imidized acrylic polymer in a non aqueous solvent. Such a resist is disclosed in U.S. Pat. No. 4,524,121. The aqueous soluble, imidized acrylic polymers, known in the art as polyglutarimides, can be dissolved in a non-reactive, non-aqueous solvent to form a positive resist that can be deposited as an adherent film on a substrate. Such films are capable of high image resolution.
Other suitable binders for positive acting photoresists include the terpolymers of an alkyl acrylate, a styrene and an acid as disclosed in U.S. Pat. No. 3,637,384 and the polyamic condensation products of an aromatic dianhydride and an aromatic di-primary amine as disclosed in U.S. Pat. No. 4,093,461.
All of the aforesaid references are incorporated herein by reference for their teachings of photoactive compounds, binders and photoresist formulations.
Photoresists of the type described above are used in integrated circuit fabrication where significant effort has been expended on increasing the resolution capabilities of the photoresists to enable a greater number of circuits to be placed on a single chip. This increase in circuit density increases the potential complexity and speed of the resulting integrated circuit.
Present techniques in optical projection printing can resolve on micron lines in photoresists with good line width control when flat, low reflectivity substrates are used. However, when exposing resists on substrates with surface topography, there are resist-controlled problems introduced by optical reflections and by resist thickness non-uniformity.
Reflection of light from the substrate photoresist interface produces variations in the light intensity in the photoresist during exposure resulting in non-uniform line width. Light can scatter from the interface into regions of the resist where no exposure was intended resulting in a narrowing of the line width. The amount of scattering and reflection will typically vary from region to region resulting in line width non-uniformity.
To eliminate the effects of chromatic aberration, monochromatic or quasi-monochromatic light is commonly used in photoresist projection techniques to expose the photoresist. Unfortunately, the effects of interface reflections on resolution is particularly significant when monochromatic or quasi-monochromatic light is used to expose the photoresist. When such light reflects from the substrate photoresist interface, the reflected light interferes with the incident light to form standing waves within the photoresist. In the case of highly reflective substrate regions, the resulting large standing wave ratio creates thin layers of under exposed resist at the standing wave minima. The under exposed layers can prevent complete photoresist development causing a jagged resist profile. Part of the reflected light also reflects back to the substrate from the top surface of the resist. Such multiple reflection of the incident light between the top and bottom surfaces of the photoresist layer result in a resonance affecting the light intensity within the resist. The time required to expose the photoresist is generally an increasing function of photoresist thickness because of the increase total amount of light required to expose an increased amount of photoresist. However, because of the resonant affect, the time of exposure also includes a harmonic component which varies between successive maximum and minimum values as the resist thickness varies through a quarter wavelength of the incident light. If the photoresist thickness is non-uniform, there will be a non-uniform exposure resulting in variable line width control.
The prior art has reduced scattering and reflection within photoresist layers by addition of dyes to photoresist compositions that absorb at or near the wavelength used to expose the photoresist. Typical dyes that have been used for this purpose include the Coumarin dyes, methyl orange and methanil yellow. These dyes absorb at the conventional exposure wavelength of 436 nm. However, it has been found that the addition of such dyes to photoresist compositions create other problems. For example, in some cases, the dye in a photoresist coating composition may cause striations in the final coating. In addition, shelf life problem may be encountered due to dye precipitation during storage. Further, because of limited solubility, such dyes can only be added to photoresist formulations in concentrations inadequate to fully attenuate reflected light. In this respect, because the dye absorbs at the activating radiation for the photoresist, the presence of the dye results in a requirement for longer exposure time or greater exposure intensity, or alternatively, dissolution rate of the photoresist during development is inhibited. For this reason even if the dye could be dissolved in the photoresist in greater concentration, the dye in greater concentration may render the photoresist inoperative.
Due to problems arising from addition of a dye directly to a photoresist formulation, the art has attempted to use dyes to absorb reflected light by means other than by direct addition to the formulation. One method is disclosed by H. A. Koury and K. V. Patel, Anti-Interference Phenomena Coating, IBM Technical Disclosure Bulletin, Vol. 13, No. 1, p. 38, June, 1970 where a thin ultraviolet light-absorbing layer containing a dye such as the aforesaid methyl orange or methanil yellow is deposited at the substrate-resist interface and is overcoated with the photoresist layer. Another method of utilizing a dye is disclosed in U.S. Pat. No. 4,362,809 where the substrate surface is covered with a bottom layer of resist containing a dye of a thickness sufficient to produce a planar surface. The top photoresist layer functions as a portable conformable mask. The two photoresist layers are selected to be sensitive to differing wavelengths of activating radiation. The top resist layer comprising the portable conformable mask is exposed and developed to produce a mask opaque to deep ultraviolet light. The bottom layer is then exposed through the mask with deep ultraviolet light and developed to produce the desired photoresist pattern. The dye in the bottom layer absorbs within the range of wavelengths used to expose the top resist layer to reduce problems due to substrate resist interface reflections.
The above cited references are incorporated herein for their teachings of dyes in resist layers.
Copending U.S. patent application Ser. No. 06/922,391 filed Oct. 23, 1986, incorporated herein by reference, and assigned to the same assignee as the subject application, is directed to a photoresist formulation containing the dye curcumin for the purpose of absorbing light within a wavelength of 400 to 460 nm, the wavelength used to form the latent image within the photoresist layer. Curcumin is said to significantly reduce reflection from a photoresist film-substrate interface during exposure of the photoresist film to activating radiation. The use of curcumin is said to be a significant improvement over other dyes capable of absorbing within the same wavelength because it is substantially more soluble than other dyes used for similar purpose. Because of enhanced solubility, the dye can be used in greater concentration than other dyes whereby it is capable of absorbing a greater quantity of reflected light. In addition, it can be used in concentrations lower than its solubility limit in the photoresist formulation thereby avoiding precipitation during storage. Further, though the dye absorbs at the proper wavelength, and does prevent reflection, it does not decrease dissolution of the resist during development to the same extent as other dyes and for this reason, can be used in higher concentration than other dyes.